1. Field of the Invention
This invention relates to a process for producing a catalyst precursor and a corresponding catalyst for manufacturing maleic anhydride by oxidizing butane.
2. Description of the Related Art
Heretofore, compounds comprising vanadium and phosphorus have been used as catalysts for processes for producing maleic anhydride by oxidizing a saturated hydrocarbon of 4 carbon atoms, usually, n-butane, with an oxygen-containing gas. It has been known that the compound comprising vanadium and phosphorus effective as the catalyst is a crystalline compound having the formula (VO).sub.2 P.sub.2 O.sub.7. In order to obtain a compound having the formula (VO).sub.2 P.sub.2 O.sub.7 as an effective component of the catalyst, usually, (V.sub.2 O.sub.5 is reduced to V.sub.2 O.sub.4 by the conventional method and the V.sub.2 O.sub.4 is reacted with phosphoric acid to form (VO).sub.2 H.sub.4 P.sub.2 O.sub.9 followed by thermal decomposition. The catalyst precursor, (VO).sub.2 H.sub.4 P.sub.2 O.sub.9, can be produced usually by reacting V.sub.2 O.sub.5 with a reducing agent and phosphoric acid in an organic solvent, particularly, in an alcohol such as isobutyl alcohol. The reaction results in precipitation of the desired product, (VO).sub.2 H.sub.4 P.sub.2 O.sub.9. This precipitation is advantageous for isolating and collecting the effective catalyst component.
U S Pat. No. 4,132,670 discloses that V.sub.2 O.sub.5 is reduced to V.sub.2 O.sub.4 in an alcohol in the presence of a dihydric alcohol and the resulting V.sub.2 O.sub.4 is reacted with phosphoric acid to form the catalyst precursor, (VO).sub.2 H.sub.4 P.sub.2 O.sub.9, so as to obtain a catalyst having the formula (VO).sub.2 P.sub.2 O.sub.7 and a large specific surface area. According to its example, glycerine is added to amyl alcohol to reduce V.sub.2 O.sub.5 to V.sub.2 O.sub.4, which is then reacted with phosphoric acid to produce the catalyst precursor. However, nothing is mentioned about its function and effect where the polyhydric alcohol is used.
U.S. Pat. No. 4,396,535 discloses a process for reducing .alpha.-VOPO.sub.4 to (VO).sub.2 P.sub.2 O.sub.7 in an organic medium, but nothing is mentioned about its function and effect where reduction is carried out by using a polyhydric alcohol as an organic medium.
EP 98,039 discloses that ethylene glycol is used as an organic medium upon reducing V.sub.2 O.sub.5, but neither concrete working examples for its effects are disclosed.
U.S. Pat. Nos. 4,365,069 and 4,448,893 disclose methods for reducing a pentavalent vanadium in the presence of an alcohol and a glycol and reacting with phosphoric acid, but any effect and concrete working examples concerning using the glycol are not disclosed.
However, the catalyst produced by thermally decomposing a precursor comprising (VO).sub.2 H.sub.4 P.sub.2 O.sub.9 has a density of 1 g/ml or less and is poor in mechanical strength. In particular, when the catalyst is used as a catalyst in a fluidized bed, its attrition-resistance is so poor that it can not withstand actual industrial operations. In conventional processes for producing catalysts for fluidized beds, a slurry prepared by suspending the catalyst precursor in water is spray-dried, and the resulting spray-dried product is thermally decomposed. However, the thus-prepared catalyst has a low density and poor attrition-resistance.
In order to solve the above problems, it has been proposed to finely pulverize the precipitated catalyst precursor to a size of 1 .mu.m or less by using a mechanical means such as a ball mill and the like. Subsequently, the pulverized precursor is suspended in water to form a slurry, which is then spray-dried and calcined (see Japanese Patent Application Laid-Open No. 55,350/84). This procedure, however, is complicated, and the attrition-resistance of the resulting catalyst is still insufficient. The surface area of the resulting catalyst is small, and when the catalyst is used for the oxidation of butane to produce maleic anhydride, the oxidation must be conducted at a temperature as high as 430.degree. to 450.degree. C. As a result, the selectivity in forming maleic anhydride is low.